This invention relates to a process and apparatus for thickening or dewatering of sludges produced from biological domestic or industrial waste water treatment processes. Sludge produced from biological treatment processes normally contains from 95 to 99.5% water. By reducing the water content, the volume and weight of the sludge will be reduced so that the cost of ultimate sludge disposal will be significantly decreased.
Currently, mechanical and mechanical/chemical processes exist for thickening biological sludges. These processes utilize belt presses, frame presses, belt thickeners, and dissolved air floatation for reducing the water content in the sludges. Almost all of these processes incorporate the addition of ionic polymers to aid in the coagulation of the sludge solid floc particles. The solids content of the thickened sludges leaving these processes varies from 30% to 4% depending upon the specific means of operation. In most cases, the higher the solids content of the thickened sludge, the greater the associated processing cost.
The concept of this invention is very similar to the theory associated with dissolved air flotation processes, the primary difference having to do with the mechanisms by which the solid sludge particles are floated. In this invention, nitrate ions (NO.sub.3 -) are added to sludge supplied from an aerobic or anaerobic treatment process. The flotation mechanism in this invention is based on the adherence of microscopic bubbles of nitrogen gas to sludge floc particles. More specifically, under anaerobic conditions, the facultative bacteria which are commonly present in waste water sludges will utilize the nitrate molecule as a source of oxygen. As the bacteria utilize the oxygen from the nitrate molecules, they release microscopic sized bubbles of nitrogen gas. The bubbles of nitrogen adhere or are trapped by the sludge floc particles, causing a reduction in density of the sludge floc, thereby causing the sludge floc particles to float. It is to be understood that with this process, no additional air or other gas need be dissolved under pressure in the liquid sludge.
As is apparent from the above description, the flotation process according to this invention is dependent upon the release of nitrogen gas. The reduction of the nitrate to nitrogen is a biological reaction, and therefore the rate of release of nitrogen is relatively slow. Accordingly, the flotation process in accordance with this invention requires approximately 16-24 hours of quiescence to maximize the sludge solids content.
In an exemplary embodiment of this invention, raw sludge is initially treated in a digester tank and thereafter pumped to at least a pair of sludge thickening tanks. At a point intermediate the digester tank and sludge thickening tanks, a dosage of nitrate is added to the sludge. Any source of nitrate (NO.sub.3 -) ions in a concentration of about 20 to about 80 mg of NO.sub.3 per liter (depending on sludge characteristics) will be effective. A solution of sodium nitrate (NaNO.sub.3) has been found to work well, although any water soluble salt may be used. The thickened sludge, with nitrate added, is maintained in the sludge thickening tanks for approximately 16 to 24 hours. During this period of time, as previously described, bacteria will utilize the oxygen associated with the nitrate molecules and will release fine bubbles of nitrogen gas. The nitrogen gas will adhere to the sludge floc particles and cause the solids to float to the surface. This will enable the residual free water, or subnate, to be removed from underneath the floated solids, by pump or by gravity, resulting in a higher sludge solids concentration.
The subnate should be drawn off at a relatively slow rate to avoid disruption of the solid float layer. The suction or draw-off piping for the subnate must also be adjustable in the sense that it can be maintained in the free water or subnate zone. This is important because, as subnate is removed, the solid float layer within the tanks drops. To this end, subnate removal piping is provided in each of the sludge thickening tanks, such that subnate outlets are arranged vertically at different levels within the tanks. Subnate may initially be removed simultaneously from each of the outlets. As the level of the combined sludge and subnate falls within the tank, the higher subnate outlets may be shut off sequentially. Control of the subnate outlets may be governed by the quality of the subnate. For example, when the subnate deteriorates to a suspended solids content greater than 200 mg/liter, the subnating process is discontinued at that level. Conventional means may be employed to monitor the quality of the subnate at levels generally corresponding to the subnate outlets.
Once all of the subnate is removed, the thickened sludge can then be pumped from the bottom of the sludge thickening tanks and thereafter pumped to, for example, tank trucks which will carry the thickened sludge to a disposal location away from the site.
In order to maintain continuity of flow, it may be desirable to recirculate a portion of the thickened sludge back to the sludge thickening tanks.
After the tanks have been emptied, or substantially emptied depending on the extent of recirculation, a new batch of nitrate-treated sludge may be pumped into the tanks and the process repeated.
Other objects and advantages of the invention will become apparent upon inspection of the drawings and the detailed description of the invention which follows.